Hand-held infrared thermometers (commonly referred to as infrared pyrometers or radiometers) have been used for many years to measure the temperature of surfaces of objects from a remote location. Their principal of operation is well known. All surfaces at a temperature above absolute zero emit heat in the form of radiated energy. This radiated energy is created by molecular motion which produces electromagnetic waves. Thus, some of the energy in the material is radiated in straight lines away from the surface of the material. The radiometer is aimed at the surface from which the measurement is to be taken, and the radiometer optical system receives the emitted radiation and focuses it upon an infrared-sensitive detector. The detector generates an electrical signal which is internally processed by the radiometer circuitry and converted into a temperature reading which is displayed.
Such radiometers are provided with sighting means which enable the users to accurately aim the radiometers. Prior radiometers commonly employ various types of laser beam sighting devices which project one or more visible laser light spots and/or patterns onto the target surface.
Single beam laser sighting devices which generate a single light spot may be used to indicate to the user the center of the target surface, i.e. the zone which is viewed by the radiometer and is determined by the field of view of the radiometer optical system. However, it is necessary to determine not only the location but also the size of this field of view on the target surface to ensure accuracy and reliability of the resultant measurement. For example, if the target surface is smaller than the field of view, or is irregularly shaped, it will not fill the entire field of view and the measurement reading will be low, i.e. in error. If the radiometer optical system is afocal, i.e. focused at infinity, its field of view will change with changes in the distance-to-target. If the optical system is focal, i.e. focused at some distance in front of the radiometer, it will be appreciated that the field of view of the optical system will be such that the size, e.g. diameter, of the field of view changes, and usually increases directly, with the distance of the radiometer from the target surface. The typical energy zone of such radiometers is defined as where 90% of the energy focused on the detector is found. Heretofore, the approximate size of the actual energy zone has been determined by the user by consulting a “distance-to-target” table, or by actual physical measurement.
Single or multiple laser beam sighting devices which are designed to generate multiple light spots or patterns may be used to indicate to the user, both the position and the outline or size of the field of view on the target surface. Sighting devices have also been proposed which generate two converging laser beams which intersect at a predetermined distance corresponding to the location of the focal point of a fixed-focus radiometer optical system, which is the location of the smallest field of view which can be resolved by the radiometer.
However, these prior radiometers or associated sighting devices are not capable of measuring the actual radiometer-to-target distance.